The invention relates to multilayer thermally imageable elements useful in lithographic printing. More particularly, this invention relates to lithographic printing plates that can be thermally imaged by imagewise exposure with a laser or a thermal printing head.
In lithographic printing, ink receptive areas are generated on the surface of a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink and the ink receptive areas accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Lithographic printing plates typically comprise a radiation-sensitive coating applied over the hydrophilic surface of a support. If after exposure to radiation the exposed regions of the coating become soluble and are removed in the developing process revealing the underlying hydrophilic surface of the support, the plate is called as a positive-working printing plate. Conversely, if exposed regions of the plate become insoluble in the developer and the unexposed regions are removed by the developing process, the plate is called a negative-working plate. In each instance, the regions of the radiation-sensitive layer (i.e., the image areas) that remain are ink-receptive and the regions of the hydrophilic surface revealed by developing process accept water, a typically fountain solution, and repel ink.
Direct digital imaging of offset printing plates, which obviates the need for exposure through a negative, is becoming increasingly important in the printing industry. Heat-sensitive imaging elements for the preparation of positive-working lithographic printing plates have been disclosed that comprise a substrate, an aqueous alkali soluble underlayer, and an infrared sensitive top layer. After infrared exposure, the exposed regions of the top layer are removable with an aqueous alkaline developer, revealing the underlying substrate. Imaging can be carried out with an infrared laser.
Thermally imageable systems have been developed in which it is believed that a thermally frangible complex is formed between a polymeric material and a solubility-suppressing component, which reversibly suppresses the solubility of the polymeric material in the developer. The solubility-suppressing component can be a separate dissolution inhibitor compound and/or the polymeric material can act as a solubility-suppressing component. After imagewise thermal exposure, the rate of dissolution of the exposed regions is greater that the rate of dissolution of the unexposed regions. The exposed regions may be removed by the developer to form an image. Such systems are disclosed in, for example, Parsons, WO 97/39894, and U.S. Pat. No. 6,280,899; West, EP 0 823 327; Miyake, EP 0 909 627; West, WO 98/42507 and U.S. Pat. No. 6,090,532; and Nguyen, WO 99/11458, and U.S. Pat. No. 6,060,217, all of which are incorporated herein by reference.
These systems have limited shelf life and can only be used with a restricted group of developers. They have only a narrow working latitude compared to conventional positive-working printing plates. These systems also have limited resistance to aggressive blanket washes, such as a UV wash.
Systems have been produced in which a developer-insoluble top layer is coated over a developer-soluble underlayer. After exposure the top layer becomes permeable to developer, which penetrates the top layer and dissolves the underlayer, revealing the hydrophilic substrate. However, the developer insoluble material in the top layer, which is removed when the underlayer is removed, causes sludging in the developer. Insoluble particles, which can adhere to the imaged and developed printing plate and adversely affect the quality of the printed image, are produced. The processor must be modified so that the insoluble particles are not circulated in the developer. Thus, a need exists for an improved thermally imageable element, useful as a lithographic printing member, that does not suffer from these disadvantages.
The invention is a multilayer thermally imageable element, useful as a precursor for a lithographic printing member. The element comprises, in order:
a) a substrate, the substrate having a hydrophilic surface;
b) an underlayer over the hydrophilic surface, the underlayer comprising a first polymeric material; and
c) a top layer over the underlayer, the top layer comprising a second polymeric material;
in which:
the top layer is ink receptive;
the top layer is insoluble in aqueous alkaline developer;
the top layer comprises at least one solubility-suppressing component;
the underlayer comprises a photothermal conversion material;
the underlayer is soluble in aqueous alkaline developer; and
the second polymeric material is soluble in aqueous alkaline developer.
The top layer is insoluble in an aqueous alkaline developer before thermal exposure but becomes removable by the aqueous alkaline developer following thermal exposure.
In a preferred embodiment, the top layer comprises a solubility-suppressing component and the second polymeric material contains phenolic hydroxyl groups. The solubility-suppressing component may be a dissolution inhibitor compound and/or the second polymeric material may act as the solubility-suppressing component. Novolac resins functionalized with sulfonate ester groups are preferred polymeric materials that function as solubility-suppressing components. Quinonediazides (i.e., compounds that contain the o-diazonaphthoquinone moiety) are preferred dissolution inhibitor compounds.
In another preferred embodiment, the underlayer comprises an absorber than absorbs radiation in the range of the imaging wavelength, typically about 800 nm to 1200 nm. In another preferred embodiment the underlayer comprises a solvent resistant underlayer or a negative-working, base soluble photosensitive composition.
In another embodiment, the invention is an exposed and developed element, which can be used as a lithographic printing member. In another embodiment, the invention is a process for forming the lithographic printing member. In still another embodiment, the invention is a method of printing using the lithographic printing member.
The thermally imageable element (sometimes referred to as a printing plate precursor) comprises a hydrophilic substrate, typically comprising an aluminum or polyester support; a radiation absorbing underlayer comprising a first polymeric material; and an ink-receptive top layer comprising a second polymeric material. Although other layers, such as radiation absorbing layers may be present, typically no other layers are present.
The top layer preferably comprises a compound that functions as a solubility-suppressing component. The solubility-suppressing component may be a separate dissolution inhibitor compound and/or the second polymeric material may also function as a solubility-suppressing component. Following exposure to radiation, top layer becomes more readily removable by an aqueous alkaline developer, i.e., more soluble or more readily dispersible in the aqueous alkaline developer. The aqueous alkaline developer removes the top layer and reveals the hydrophilic surface of the substrate in the exposed regions. The lithographic printing member thus formed has excellent properties, including the absence of sludging of the developer.
The hydrophilic substrate, i.e., the substrate that comprises at least one hydrophilic surface, comprises a support, which may be any material conventionally used to prepare lithographic printing plates. The support is preferably strong, stable and flexible. It should resist dimensional change under conditions of use so that color records will register in a full-color image. Typically, it can be any self-supporting material, including polymeric films, ceramics, metals, or stiff papers, or a lamination of any of these materials. Paper supports are typically xe2x80x9csaturatedxe2x80x9d with polymerics to impart water resistance, dimensional stability and strength.
Metal supports include aluminum, zinc, titanium, and alloys thereof. A preferred metal support is an aluminum sheet. The surface of the aluminum sheet may be treated by techniques known in the art, including physical graining, electrochemical graining, chemical graining, and anodizing, and then conditioned by chemical means, for example by treatment with water, a solution of phosphate or silicate salt, or a polycarboxylic acid to produce the hydrophilic surface.
If the surface is roughened, the average roughness Ra is preferably in the range 0.1 xcexcm to 0.8 xcexcm. Roughened substrates in which the surface has a surface roughness of 0.1 xcexcm to 2 xcexcm are disclosed in Bhambra, WO97/19819 (PCT/GB96/02883); Bhambra, WO98/52769 (PCT/GB98/01500); and Bhambra, WO98/52768 (PCT/GB/98/01496). In these substrates the support is coated with a hydrophilic layer that comprises a mixture of two particulate materials, preferably alumina and titanium dioxide. The mean particle size of the alumina particles is preferably in the range of 1 xcexcm to 5 xcexcm; the mean particle size of the titanium dioxide particles is preferably in the range of 0.1 xcexcm to 0.5 xcexcm.
Useful polymeric films include polyester films (such as MYLAR(copyright) polyethylene terephthalate film sold by E.I. du Pont de Nemours and Co., Wilmington, Del., and polyethylene naphthanate). A preferred polymeric film is polyethylene terephthalate.
The substrate may consist only of the support, or it may additionally comprise one or more optional subbing and/or adhesion layers. Typically, polymeric films contain a sub-coating on one or both surfaces to modify the surface characteristics to enhance the hydrophilicity of the surface, to improve adhesion to subsequent layers, to improve planarity of paper substrates, and the like. The nature of this layer or layers depends upon the substrate and the composition of subsequent coated layers. Examples of subbing layer materials are adhesion promoting materials, such as alkoxysilanes, aminopropyltriethoxysilane, glycidoxypropyltriethoxysilane and epoxy functional polymers, as well as conventional subbing materials used on polyester bases in photographic films.
The back side of the substrate (i.e., the side opposite the underlayer and top layer) may be coated with an antistatic agent and/or a slipping layer or matte layer to improve handling and xe2x80x9cfeelxe2x80x9d of the imageable element.
The support should be of sufficient thickness to sustain the wear from printing and be thin enough to wrap around a printing form. Polyethylene terephthalate or polyethylene naphthanate, typically has a thickness of from about 100 to about 310 xcexcm, preferably about 175 xcexcm. Aluminum sheet typically has a thickness of from about 100 to about 600 xcexcm.
The underlayer, or first layer, is over the hydrophilic surface of the hydrophilic substrate. After imaging, it is removed by the aqueous alkaline developer to expose the underlying hydrophilic surface of the substrate. It is preferably soluble in the aqueous alkaline developer to prevent sludging of the developer. Preferably it is soluble in a wholly aqueous developer, i.e. one that does not include added organic solvents.
The underlayer comprises a first polymeric material. The first polymeric material is soluble in an aqueous alkaline developer. In addition, the first polymeric material should be insoluble in the solvent used to coat the top layer so that the top layer can be coated over the underlayer without dissolving the underlayer.
Polymeric materials useful as the first polymeric material include those that contain an acid and/or phenolic functionality, and mixtures of such materials. Useful polymeric materials include carboxy functional acrylics, vinyl acetate/crotonate/vinyl neodecanoate copolymers, styrene maleic anhydride copolymers, phenolic resins, maleated wood rosin, and combinations thereof.
Particularly useful polymeric materials are copolymers that comprise N-substituted maleimides, especially N-phenylmaleimide; polyvinylacetals; methacrylamides, especially methacylamide; and acrylic and/or methacrylic acid, especially methacrylic acid. More preferably two functional groups are present in the polymeric material, and most preferably all three functional groups are present in the polymeric material. The preferred polymeric materials of this type are copolymers of N-phenylmaleimide, methacrylamide, and methacrylic acid, more preferably those that contain about 25 to about 75 mol %, preferably about 35 to about 60 mol % of N-phenylmaleimide; about 10 to about 50 mol %, preferably about 15 to about 40 mol % of methacrylamide; and about 5 to about 30 mol %, preferably about 10 to about 30 mol %, of methacrylic acid. Other hydrophilic monomers, such as hydroxyethyl methacrylate, may be used in place of some of all of the methacrylamide. Other aqueous alkaline soluble monomers, such as acrylic acid, may be used in place of some or all of the methacrylic acid.
These polymeric materials are soluble in aqueous alkaline developers. In addition they are soluble in methyl lactate/methanol/dioxolane (15:42.5:42.5 wt %) mixture, which can be used as the coating solvent for the underlayer. However, they are poorly soluble in solvents such as acetone, which can be used as solvents to coat the top layer on top of the underlayer without dissolving the underlayer. These polymeric materials are typically resistant to washes with 80 wt % diacetone alcohol/20 wt % water.
Another group of preferred polymeric materials for the first polymeric material are aqueous alkaline developer soluble copolymers that comprise a monomer that has a urea bond in its side chain (i.e., a pendent urea group), such are disclosed in Ishizuka, U.S. Pat. No. 5,731,127, incorporated herein by reference. These copolymers comprise about 10 to 80 wt %, preferably about 20 to 80 wt %, of one of more monomers represented by the general formula:
[CH2=C(R)xe2x80x94CO2xe2x80x94Xxe2x80x94NHxe2x80x94COxe2x80x94NHxe2x80x94Yxe2x80x94Z], 
in which R is xe2x80x94H or xe2x80x94CH3; X is a bivalent linking group; Y is a substituted or unsubstituted bivalent aromatic group; and Z is xe2x80x94OH, xe2x80x94COOH, or xe2x80x94SO2NH2.
R is -preferably CH3. Preferably X is a substituted or unsubstituted alkylene group, substituted or unsubstituted phenylene [C6H4] group, or substituted or unsubstituted naphthalene [C10H6] group; such as xe2x80x94(CH2)nxe2x80x94, in which n is 2 to 8; 1-,2-, 1,3-, and 1,4-phenylene; and 1,4-, 2,7-, and 1,8-naphthalene. More preferably X is unsubstituted and even more preferably n is 2 or 3; most preferably X is xe2x80x94(CH2CH2)xe2x80x94. Preferably Y is a substituted or unsubstituted phenylene group or substituted or unsubstituted naphthalene group; such as 1,2-, 1,3-, and 1,4-phenylene; and 1,4-, 2,7-, and 1,8-naphthalene. More preferably Y is unsubstituted, most preferably unsubstituted 1,4-phenylene. Z is xe2x80x94OH, xe2x80x94COOH, or xe2x80x94SO2NH2, preferably xe2x80x94OH. A preferred monomer is:
[CH2=C(CH3)xe2x80x94CO2xe2x80x94CH2CH2xe2x80x94NHxe2x80x94COxe2x80x94NHxe2x80x94pxe2x80x94C6H4xe2x80x94Z], 
in which Z is xe2x80x94OH, xe2x80x94COOH, or xe2x80x94SO2NH2, preferably xe2x80x94OH.
In the synthesis of a copolymer, one or more of the urea group containing monomers may be used. The copolymers also comprise 20 to 90 wt % other polymerizable monomers, such as maleimide, acrylic acid, methacrylic acid, acrylic esters, methacrylic esters, acrylonitrile, methacrylonitrile, acrylamides, and methacrylamides. A copolymer hat comprises in excess of 60 mol % and not more than 90 mol % of acrylonitrile and/or methacrylonitrile in addition to acrylamide and/or methacrylamide provides superior physical properties. More preferably the alkaline soluble copolymers comprise 30 to 70 wt % urea group containing monomer; 20 to 60 wt % acrylonitrile or methacrylonitrile, preferably acrylonitrile; and 5 to 25 wt % acrylamide or methacrylamide, preferably methacrylamide. These polymeric materials are typically resistant to washes with 80 wt % 2-butoxyethanol/20 wt % water.
The polymeric materials described above are soluble in aqueous alkaline developers. In addition, they are soluble in polar solvents, such as ethylene glycol monomethyl ether, which can be used as the coating solvent for the underlayer. However, they are poorly soluble in less polar solvents, such as 2-butanone (methyl ethyl ketone), which can be used as a solvent to coat the top layer over the underlayer without dissolving the underlayer.
Both these groups of polymeric materials can be prepared by methods, such as free radical polymerization, well known to those skilled in the art. Synthesis of the aqueous alkaline soluble copolymers that have urea bonds in their side chains is disclosed, for example, in Ishizuka, U.S. Pat. No. 5,731,127.
Other aqueous alkaline developer soluble polymeric materials may be useful in the underlayer. Derivatives of methyl vinyl ether/maleic anhydride copolymers that contain an N-substituted cyclic imide moiety and derivatives of styrene/maleic anhydride copolymers that contain an N-substituted cyclic imide moiety may be useful if they have the required solubility characteristics. These copolymers can be prepared by reaction of the maleic anhydride copolymer with an amine, such as p-aminobenzenesulfonamide, or p-aminophenol, followed by ring closure by acid.
Another group of polymeric materials that are useful in the underlayer include aqueous alkaline developer soluble copolymers that comprise about 10 to 90 mol % of a sulfonamide monomer unit, especially those that comprise N-(p-aminosulfonylphenyl)methacrylamide, N-(m-aminosulfonylphenyl)methacrylamide N-(o-aminosulfonylphenyl)methacrylamide, and/or the corresponding acrylamide. Useful alkaline developer soluble polymeric materials that comprise a pendent sulfonamide group, their method of preparation, and monomers useful for their preparation, are disclosed in Aoshima, U.S. Pat. No. 5,141,838, incorporated herein by reference. Particularly useful polymeric materials comprise (1) the sulfonamide monomer unit, especially N-(p-aminosulfonylphenyl)methacrylamide; (2) acrylonitrile and/or methacrylonitrile; and (3) methyl methacrylate and/or methyl acrylate. These polymeric materials are typically resistant to washes with 80 wt % 2-butoxyethanol/20 wt % water.
Combinations of alkaline developer soluble polymeric materials may be used in the underlayer to provide improved chemical resistance, i.e., resistance to both fountain solution and to aggressive washes. A combination of a polymeric material that is resistant to 80 wt % diacetone alcohol/20 wt % water, which tests resistance to a UV wash, with a polymeric material that is resistant to 80 wt % 2-butoxyethanol/20 wt % water, which tests resistance to alcohol sub fountain solution, surprisingly produces a layer that shows good resistance to both solvent mixtures. Preferably, the first polymeric material has a one-minute soak loss of less than about 20%, more preferably less than about 10%, and most preferably less than about 5% in 80 wt % diacetone alcohol/20 wt % water, and the second polymeric material has a one-minute soak loss of less than about 20%, more preferably less than about 10%, and most preferably less than about 10%, in 80 wt % 2-butoxyethanol/20 wt % water. One-minute soak loss is measured by coating a layer of the polymeric material on a substrate, typically at a coating weight of about 1.5 g/m2, soaking the coated substrate in the appropriate solvent for one minute at room temperature, drying the coated substrate, and measuring the weight loss as a percent of the total weight of polymeric material present on the substrate.
The ability of an underlayer to withstand both fountain solution and aggressive washes can be estimated by a chemical resistance parameter (CRP), defined as follows:
CRP=[(100xe2x88x92a)(100xe2x88x92b)]/104 
in which:
a is the one minute % soak loss in 80 wt % diacetone alcohol/20 wt % water; and
b is the one minute % soak loss in 80 wt % 2-butoxyethanol/20 wt % water.
The chemical resistance parameter should be greater than about 0.4, preferably greater than about 0.5, more preferably greater than about 0.6. In favorable cases a chemical resistance parameter of at least about 0.65 can be obtained. The one-minute soak loss in each solvent should be less than about 60%, preferably less than about 40%, and more preferably less than about 35%. Preferably, the one-minute soak loss should be less than about 60%, preferably less than about 40%, and more preferably less than about 35%, in one solvent and less than about 40%, more preferably less than about 30%; and more preferably less than about 20%, and most preferably less than about 10% in the other solvent.
Combination of (1) a copolymer that comprises N-substituted maleimides, especially N-phenylmaleimide; methacrylamides, especially methacylamide; and acrylic and/or methacrylic acid, especially methacrylic acid (2) with an alkaline soluble copolymer that comprises a urea in its side chain or with an alkaline soluble copolymer that comprises 10 to 90 mol % of a sulfonamide monomer unit, especially one that comprise N-(p-aminosulfonylphenyl)methacrylamide, N-(m-aminosulfonylphenyl)methacrylamide N-(o-aminosulfonylphenyl)methacrylamide, and/or the corresponding acrylamide, is especially advantageous. One or more other polymeric materials, such as novolac resins, may also be present in the combination. Preferred other polymeric materials, when present, are novolac resins.
When a combination of polymeric materials is used, the underlayer typically comprises about 10% to about 90% by weight of the polymeric material that is resistant to 80 wt % diacetone alcohol/20 wt % water, and about 10% to about 90% by weight of the polymeric material that is resistant to 80 wt % 2-butoxyethanol/20 wt % water, based on the total weight these polymeric materials in the underlayer. Preferably the underlayer comprises about 40% to about 85% by weight of the polymeric material that is resistant to 80 wt % diacetone alcohol/20 wt % water and about 15% to about 60% of the polymeric material that is resistant to 80 wt % 2-butoxyethanol/20 wt % water, based on the total weight the first and second polymeric materials in the underlayer. The first and second polymeric materials together typically comprise at least about 50 wt %, preferably at least about 60 wt %, and more preferably at least about 65 wt %, of the underlayer, based on total weight of the materials in the underlayer. Up to about 20 wt %, preferably about 1 to about 20 wt %, of other polymeric materials may be present in the underlayer, based on the total amount of all the polymeric materials in the underlayer.
The underlayer may comprise a negative-working, base soluble photosensitive composition. Such compositions are often referred to as xe2x80x9cphotohardenable compositionsxe2x80x9d or xe2x80x9cphotoinsolubilizable compositionsxe2x80x9d because they become developer insoluble on irradiation. Typically these compositions comprise materials that undergo photocrosslinking, photodimerization, and/or photopolymerization on exposure to actinic radiation, typically ultraviolet light. Photohardenable compositions produce printing plates with high press life and resistance to press room chemicals. Negative working systems are discussed, for example, in Chapter 2 of Photoreactive Polymers: the Science and Technology of Resists, A. Reiser, Wiley, New York, 1989, pp. 22-64.
The underlayer may comprise a negative-working diazonium-containing composition. Typically the diazonium containing compound is a diazonium polycondensation product. Diazonium polycondensation products well known to these skilled in the art. They may be prepared, for example, by condensation of a diazo monomer, such as is described in Toyama, U.S. Pat. No. 4,687,727 with a condensation agent, such as formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, isobutyraldehyde or benzaldehyde. Furthermore, mixed condensation products are used which, apart from the diazonium salt units, comprise other non-photosensitive units which are derived from condensable compounds, in particular from aromatic amines, phenols, phenol ethers, aromatic thioethers, aromatic hydrocarbons, aromatic heterocycles or organic acid amides. Especially advantageous examples of diazonium polycondensation products are reaction products of diphenylamine-4-diazonium salts, optionally having a methoxy group in the phenyl group bearing the diazo group, with formaldehyde or 4,4xe2x80x2-bis-methoxymethyl diphenyl ether. Aromatic sulfonates such as 4-tolylsulfonate or mesitylene sulfonate, tetrafluoroborate, hexafluorophosphate, hexafluoroantimonate and hexafluoroaresenate are particularly suitable as anions of these diazo resins. The diazonium polycondensation product is preferably present in the photosensitive mixtures in an amount of from 3 to 60 wt %.
Numerous binders are known. On such system is described in Baumann, U.S. Pat. No. 5,700,619, the disclosure of which is incorporated herein by reference. The binder is an acetalized polyvinyl alcohol, with pendent carboxyl groups.
The underlayer may comprise a photopolymerizable composition. Preferred photosensitive compositions are photopolymerizable compositions, which comprise one of monomers, one or more binders, and one or more photoinitiator systems. Other components conventionally added to photopolymerizable compositions can be present to modify the physical properties of the film. Such components include: for example, plasticizers, thermal stabilizers, adhesion modifiers, coating aids, and release agents. Nonionic surfactants, for example, may be added to the photopolymerizable composition as coating aids. Such systems are well known in art, and are discussed, for example, in Photopolymers: Radiation Curable Imaging Systems, B. M. Monroe in Radiation Curing: Science and Technology, S. P. Pappas, Ed., Plenum, New York, 1992, pp. 399-440.
Photopolymerizable compositions comprise at least one ethylenically unsaturated compound that undergoes free-radical initiated polymerization, generally known as a monomer. The monomers are typically multifunctional, i.e., they comprise more than one ethylenically unsaturated, free radical polymerizable group. Typical multifunctional monomers are unsaturated esters of alcohols, preferably acrylate and methacrylate esters of polyols, such as, trimethylolpropane triacrylate and trimethacrylate, pentaerythritol triacrylate and trimethacrylate, pentaerythritol tetraacrylate and tetramethacrylate, ethoxylated-trimethylolpropane triacrylate and trimethacrylate, glycerolpropoxy triacrylate and trimethacrylate, ethyleneglycol diacrylate and dimethacrylate, tripropyleneglycol diacrylate and dimethacrylate, and tetraethyleneglycol diacrylate and dimethacrylate. Oligomers and/or prepolymers, such as urethane acrylate and methacrylate, epoxide acrylate and methacrylate, polyester acrylate and methacrylate, polyether acrylate and methacrylate or unsaturated polyester resins, may also be used. Numerous other unsaturated monomers polymerizable by free-radical initiated polymerization and useful in photopolymerizable compositions are known to those skilled in the art.
The composition comprises at least one preformed macromolecular polymeric material, generally know as a binder. In general, the binder should be soluble or swellable in the coating solvent and compatible with the other components of the photopolymerizable system. Representative binders are poly(methyl methacrylate) and copolymers of methyl methacrylate with other alkyl acrylates, alkyl methacrylates, methacrylic acid, and/or acrylic acid. Numerous other binders useful in photopolymerizable compositions are known to those skilled in the art.
When the material is to be cured by irradiation with ultraviolet or visible radiation, a free radical generating, initiating system activatable by ultraviolet or visible radiation, known as a photoinitiating system, may be present to facilitate polymerization of the polymerizable monomers. The photoinitiator system absorbs in ultraviolet and/or visible regions of the spectrum, i.e., in the range of from 300 to 800 nm, preferably in the ultraviolet, i.e. 300 nm to 400 nm.
The photoinitiating system may be a single compound or a mixture of compounds. Suitable photoinitiating systems are disclosed in xe2x80x9cPhotoinitiators for Free-Radical-Initiated Photoimaging Systems,xe2x80x9d by B. M. Monroe and G. C. Weed, Chem. Rev., 93, 435-448 (1993) and in xe2x80x9cFree Radical Polymerizationxe2x80x9d by K. K. Dietliker, in Chemistry and Technology of UV and EB Formulation for Coatings, Inks, and Paints, P. K. T. Oldring, ed, SITA Technology Ltd., London, 1991, Vol. 3, pp. 59-525. Typical free radical photoinitiating compounds include benzophenone; 2-hydroxy-2-methyl-1-phenylpropan-1-one (Darocur(copyright) 1173); 2,4,6-trimethylbenzolyl-diphenylphosphine oxide (Lucerin(copyright) TPO); 2,2-dimethoxy-2-phenyl-acetophenone (benzildimethyl ketal, BDK, Irgacure(copyright) 651, Lucerin(copyright) BDK); 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropanone-1 (Irgacure(copyright) 907); 1-hydroxycyclohexylphenyl ketone (HCPK, Irgacure(copyright) 184); bis(2,6-dimethoxybenzolyl)-2,4,4-trimethyl-pentylphosphine oxide; and combinations thereof. Mixed photoinitiators include a 50:50 mixture of 2-hydroxy-2-methyl-1-phenylpropan-1-one and 2,4,6-trimethylbenzolyl-diphenylphosphine oxide (Darocur(copyright) 4265); and a 25:75 mixture of bis(2,6-dimethoxybenzolyl)-2,4,4-trimethylpentyl-phosphine oxide and 2-hydroxy-2-methyl-1-phenylpropan-1-one (CGI 1700).
A hybrid system comprising a combination of a diazonium polycondensation product and a free polymerizable system may be advantageous for certain applications. The compositions of such hybrid systems preferably comprise 1 to 50% diazonium polycondensation products, 0.5 to 20% photoinitiators as well as 5 to 80% free radical polymerizable components.
Photocrosslinkable systems typically comprise at least one binder and a photoactivated at least bifunctional crosslinking agent that crosslinks the binder on irradiation. Organic azides, which are believed to form nitrenes on irradiation, have been used to crosslink binders. Diazido compounds, such as the disulfonated derivative of 4,4xe2x80x2-diazidostilbene, are preferred azides for photocrosslinking.
Photodimerizable systems comprise a binder that forms a crosslink on irradiation. Photocrosslinkble binders include, for example, the polyvinyl alcohols functionalized with N-alkyl styrylpyridinium or N-alkyl styrylquinolinium groups, such as are described in, for example, Ichimura, U.S. Pat. Nos. 4,272,620; 4,287,335; 4,339,524; 4,564,580; and 4,777,114.
The underlayer absorbs radiation, preferably radiation in the range of about 800 nm to 1200 nm, the range of radiation commonly used for imaging thermally imageable elements. An absorber, sometimes referred to as xe2x80x9ca photothermal conversion materialxe2x80x9d is present in the underlayer. Photothermal conversion materials absorb radiation and convert it to heat. Photothermal conversion materials may absorb ultraviolet, visible, and/or infrared radiation and convert it to heat. Although the first polymeric material may itself comprise an absorbing moiety, i.e., be a photothermal conversion material, typically the photothermal conversion material is a separate compound.
The imaging radiation absorber may be either a dye or pigment, such as a dye or pigment of the squarylium, merocyanine, indolizine, pyrilium or metal diothiolene class. Examples of absorbing pigments are Projet 900, Projet 860 and Projet 830 (all available from the Zeneca Corporation). Carbon black pigments may also be used. Because of their wide absorption bands, carbon black-based plates can be used with multiple infrared imaging devices having a wide range of peak emission wavelengths.
Dyes, especially dyes that are soluble in the aqueous alkaline developer, are preferred to prevent sludging of the developer by insoluble material. The dye may be chosen, for example, from indoaniline dyes, oxonol dyes, porphyrin derivatives, anthraquinone dyes, merostyryl dyes, pyrylium compounds and sqarylium derivatives. Absorbing dyes are disclosed in numerous disclosures and patent applications in the field, for example, Nagasaka, EP 0,823,327; Van Damme, EP 0,908,397; DeBoer, U.S. Pat. No. 4,973,572; Jandrue, U.S. Pat. No. 5,244,771; and Chapman, U.S. Pat. No. 5,401,618, all of which are incorporated herein by reference. Examples of useful absorbing dyes include, ADS-830A and ADS-1064 (both available from American Dye Source, Montreal, Canada), EC2117 (available from FEW, Wolfen, Germany), Cyasorb IR 99 and Cyasorb IR 165 (both available from Glendale Protective Technology), Epolite IV-62B and Epolite 111-178 (both available from the Epoline), PINA-780 (available from the Allied Signal Corporation), SpectraIR 830A and SpectraIR 840A (both available from Spectra Colors).
The amount of imaging radiation absorber in the underlayer is generally sufficient to provide an optical density of at least 0.05, and preferably, an optical density of from about 0.5 to about 2 at the imaging wavelength. As is well known to those skilled in the art, the amount of absorber required to produce a particular optical density can be determined from the thickness of the underlayer and the extinction coefficient of the absorber at the wavelength used for imaging using Beers law. Typically the underlayer comprises at least about 0.1 wt % of imaging radiation absorber, and preferably from about 1 to about 30 wt % of absorber.
The top layer, or second layer, comprises a second polymeric material, which is ink-receptive and dissolves in an aqueous alkaline developer. However, the top layer is insoluble in aqueous alkaline developer prior to imaging, but becomes soluble in aqueous alkaline developer following imaging. Second polymeric materials that are water insoluble, but dissolve in an aqueous alkaline developer, are used to prevent sludging of the developer. Typically the underlayer absorbs imaging radiation, i.e., infrared radiation and radiation in the range of 600 nm to 1200 nm, especially in the range of 800 nm to 1200 nm, and the top layer does not substantially absorb imaging radiation, i.e., infrared radiation and radiation in the range of 600 nm to 1200 nm, especially in the range of 800 nm to 1200 nm.
Polymers that contain phenolic hydroxyl groups, i.e., phenolic resins, are preferred. Preferably the polymeric material is a light-stable, water-insoluble, aqueous alkaline developer-soluble, film-forming polymeric material that has a multiplicity of phenolic hydroxyl groups, either on the polymer backbone or on pendant groups. Phenolic groups impart aqueous alkaline developer solubility to the top layer and are also believed to form a thermally frangible complex with the solubility-suppressing component. Novolac resins, resol resins, acrylic resins that contain pendent phenol groups, and polyvinyl phenol resins are preferred phenolic resins. Novolac resins are more preferred.
Novolac resins are commercially available and are well known to those skilled in the art. They are typically prepared by the condensation reaction of a phenol, such as phenol, m-cresol, o-cresol, p-cresol, etc, with an aldehyde, such as formaldehyde, paraformaldehyde, acetaldehyde, etc. or ketone, such as acetone, in the presence of an acid catalyst. The weight average molecular weight is typically about 1,000 to 15,000. Typical novolac resins include, for example, phenol-formaldehyde resins, cresol-formaldehyde resins, phenol-cresol-formaldehyde resins, p-t-butylphenol-formaldehyde resins, and pyrogallol-acetone resins. Particularly useful novolac resins are prepared by reacting m-cresol, mixtures of m-cresol and p-cresol, or phenol with formaldehyde using conventional conditions.
Other useful phenolic resins include polyvinyl compounds having phenolic hydroxyl groups. Such compounds include, for example, polyhydroxystyrenes and copolymers containing recurring units of a hydroxystyrene, and polymers and copolymers containing recurring units of substituted hydroxystyrenes.
The top layer preferably comprises a compound that functions as a solubility-suppressing component for the second polymeric material. While not being bound by any theory or explanation, it is believed that solubility-suppressing components are xe2x80x9creversible insolubilizers,xe2x80x9d i.e., compounds that reversibly suppress the solubility of the second polymeric material in the aqueous alkaline developer. Solubility-suppressing components have polar functional groups that are believed to act as acceptor sites for hydrogen bonding with the hydroxyl groups present in the second polymeric material. The acceptor sites comprise atoms with high electron density, preferably selected from electronegative first row elements, especially carbon, nitrogen, and oxygen. Solubility-suppressing components that are soluble in the aqueous alkaline developer are preferred.
The solubility-suppressing component may be a separate dissolution inhibitor compound. Alternatively, or additionally, the polymeric material may contain polar groups in addition to phenolic groups and, thus, function as both the polymeric material and the solubility-suppressing component. Useful dissolution inhibitor compounds are disclosed in West, U.S. Pat. No. 5,705,308; Bennett, WO97/07986 [PCT/GB96/01973], West, EP 0 832 327 and U.S. Pat. No. 6,060,222, all of which are incorporated herein by reference.
Solubility-suppressing components are believed to reversibly reduce the rate at which the second polymeric material dissolves in an aqueous alkaline developer. In general, such compounds should have an xe2x80x9cinhibition factorxe2x80x9d of at least 0.5, and preferably at least 5. Inhibition factors for given compounds can be readily measured using the procedure described by Shih et al, Macromolecules, 27, 3330 (1994). The inhibition factor is the slope of the line obtained by plotting the log of the development rate as a function of inhibitor concentration in the coating. Development rates are conveniently measured by laser interferometry, as described by Meyerhofer, IEEE Trans. Electron Devices, ED-27, 921 (1980).
Useful polar groups include, for example, diazo groups; diazonium groups; keto groups; sulfonic acid ester groups; phosphate esters groups; triarylmethane groups; onium groups, such as sulfonium, iodonium, and phosphonium; groups in which a nitrogen atom is incorporated into a heterocyclic ring; and groups that contain a positively charged atom, especially a positively charged nitrogen atom, typically a quaternized nitrogen atom, i.e., ammonium groups. Compounds containing other polar groups, such as ether, amine, azo, nitro, ferrocenium, sulfoxide, sulfone, and disulfone may also be useful as solubility-suppressing components. Monomeric or polymeric acetals having recurring acetal or ketal groups, monomeric or polymeric ortho carboxylic acid esters having at least one ortho carboxylic acid ester or amide group, enol ethers, N-acyliminocarbonates, cyclic acetals or ketals, xcex2-ketoesters or xcex2-ketoamides may also be useful as solubility-suppressing components. Compounds that contain aromatic groups, such as phenyl, substituted phenyl such as p-methylphenyl, and naphthyl, are especially useful.
Compounds that contain a diazo group that are useful as dissolution inhibitor compounds include, for example, compounds that contain the o-diazonaphthoquinone moiety (i.e., quinonediazides), preferably compounds that comprise an o-diazonaphthoquinone moiety attached to a ballasting moiety that has a molecular weight of at least 1500, but less than about 5000. Typically these compounds are prepared by the reaction of a 1,2-naphthoquinone diazide having a halogenosulfonyl group, typically a sulfonylchloride group, at the 4- or 5-position with a mono- or poly-hydroxyphenyl compound, such as mono- or poly-hydroxy benzophenone. Preferred reactive compounds are the sulfonyl chloride or esters; the sulfonyl chlorides are most preferred. These compounds are discussed, for example, in Chapter 5 of Photoreactive Polymers: the Science and Technology of Resists, A. Reiser, Wiley, New York, 1989, pp. 178-225.
Useful compounds include, but are not limited to: 2,4-bis(2-diazo-1,2-dihydro-1-oxo-5-naphthalenesulfonyloxy)benzophenone; 2-di-azo-1,2-dihydro-1-oxo-5-naphthalenesulfonyloxy-2,2-bishydroxyphenylpropane monoester; the hexahydroxybenzophenone hexaester of 2-diazo-1,2-dihydro-1-oxo-5-naphthalenesulfonic acid; 2,2xe2x80x2-bis(2-diazo-1,2-dihydro-1-oxo-5-naphthalenesulfonyloxy)biphenyl; 2,2xe2x80x2,4,4xe2x80x2-tetrakis(2-diazo-1,2-dihydro-1-oxo-5-naphthalenesulfonyloxy)biphenyl; 2,3,4-tris(2-diazo-1,2-dihydro-1-oxo-5-naphthalenesulfonyloxy)benzophenone; 2,4-bis(2-diazo-1,2-dihydro-1-oxo-4-naphthalenesulfonyloxy)benzophenone; 2-diazo-1,2-dihydro-1-oxo-4-naphthalenesulfonyloxy-2,2-bis-hydroxyphenylpropane monoester; the hexahydroxybenzophenone hexaester of 2-diazo-1,2-dihydro-1-oxo-4-naphthalenesulfonic acid; 2,2 xe2x80x2-bis(2-diazo-1,2-dihydro-1-oxo-4-naphthalenesulfonyloxy)biphenyl; 2,2xe2x80x2,4,4xe2x80x2-tetrakis(2-diazo-1,2-dihydro-1-oxo-4-naphthalenesulfonyloxy)biphenyl; 2,3,4-tris(2-diazo-1,2-dihydro-1-oxo-4-naphthalenesulfonyloxy)benzophenone; and others known in the art, for example, those described in Mizutani, U.S. Pat No. 5,143,816.
The phenolic resin may be derivatized with an o-diazonaphthoquinone moiety. Polymeric diazonaphthoquinone compounds include derivatized resins formed by the reaction of a reactive derivative that contains diazonaphthoquinone moiety and a polymeric material that contains a suitable reactive group, such as a hydroxyl or amino group. Suitable polymeric materials for forming these derivatized resins include the novolac resins, resole resins, polyvinyl phenols, acrylate and methacrylate copolymers of hydroxy-containing monomers such as vinyl phenol and 2-hydroxyethyl methacrylate, polyvinyl alcohol, etc. Representative reactive derivatives include sulfonic and carboxylic acid, ester or amide derivatives of the diazonaphthoquinone moiety. Derivitization of phenolic resins with compounds that contain the diazonaphthoquinone moiety is well known in the art and is described, for example, in West, U.S. Pat. Nos. 5,705,308, and 5,705,322. An example of a resin derivatized with a compound that comprises a diazonaphthoquinone moiety is P-3000, a naphthoquinone diazide of a pyrogallol/acetone resin (available from PCAS, France). These derivatized polymeric materials can act as both the second polymeric material and a solubility-suppressing component. They can be used alone in the top layer, or they can be combined with other polymeric materials and/or solubility-suppressing components.
Compounds that contain a positively charged (i.e., quaternized) nitrogen atom useful as dissolution inhibitor compounds include, for example, tetraalkyl ammonium compounds, quinolinium compounds, benzothiazolium compounds, pyridinium compounds, and imidazolium compounds. Representative tetraalkyl ammonium dissolution inhibitor compounds include tetrapropyl ammonium bromide; tetraethyl ammonium bromide; tetrapropyl ammonium chloride; and trimethylalkyl ammonium chlorides and trimethylalky ammonium bromides, such as trimethyloctyl ammonium bromide and trimethyldecyl ammonium chloride. Representative triarylmethane dyes dissolution inhibitor compounds include ethyl violet, crystal violet, malachite green, brilliant green, Victoria blue B, Victoria blue R, and Victoria pure blue BO.
Quaternized heterocyclic compounds are useful as dissolution inhibitors. Representative imidazolium compounds include Monazoline C, Monazoline O, Monazoline CY, and Monazoline T, all of which are manufactured by Mona Industries. Representative quinolinium dissolution inhibitor compounds include 1-ethyl-2-methyl quinolinium iodide, 1-ethyl-4-methyl quinolinium iodide and cyanine dyes that comprise a quinolinium moiety such as Quinoldine Blue. Representative benzothiazolium compounds include 3-ethyl-2(3H)-benzothiazolylidene)-2-methyl-1-(propenyl)benzothiazolium cationic dyes and 3-ethyl-2-methyl benzothiazolium iodide. Suitable pyridinium dissolution inhibitor compounds include cetyl pyridinium bromide and ethyl viologen dications.
Diazonium salts are useful as dissolution inhibitor compounds and include, for example, substituted and unsubstituted diphenylamine diazonium salts, such as methoxy-substituted diphenylamine diazonium hexafluoroborates. These compounds are particularly useful in non-preheat plates.
Representative sulfonic acid esters useful as dissolution inhibitor compounds include ethyl benzene sulfonate, n-hexyl benzene sulfonate, ethyl p-toluene sulfonate, t-butyl p-toluene sulfonate, and phenyl p-toluene sulfonate. Representative phosphate esters include trimethyl phosphate, triethyl phosphate, and tricresyl phosphate. Useful sulfones include those with aromatic groups, such as diphenyl sulfone. Useful amines include those with aromatic groups, such as diphenyl amine and triphenyl amine.
Keto containing compounds useful as dissolution inhibitor compounds include, for example, aldehydes; ketones, especially aromatic ketones; and carboxylic acid esters. Representative aromatic ketones include xanthone, flavanone, flavone, 2,3-diphenyl-1-indenone, 1xe2x80x2-(2xe2x80x2-acetonaphthonyl)benzoate, xcex1- and xcex2-naphthoflavone, 2,6-diphenyl-4H-pyran4-one and 2,6-diphenyl-4H-thiopyran-4-one. Representative carboxylic acid esters include ethyl benzoate, n-heptyl benzoate, phenyl benzoate.
A preferred group of dissolution inhibitor compounds are those that are also dyes, especially triarylmethane dyes such as ethyl violet. These compounds can also act as contrast dyes, which distinguishes the unimaged regions from the imaged regions in the developed imageable element.
When a dissolution inhibitor compound is present in the top layer, its amount can vary widely, but generally it is at least about 0.1 wt %, typically about 0.5 wt % to about 30 wt %, preferably about 1 wt % to 15 wt %, based on the total dry composition weight of the layer.
Alternatively, or additionally, the second polymeric material can comprise polar groups that act as acceptor sites for hydrogen bonding with the hydroxy groups present in the polymeric material and, thus, act as a solubility-suppressing component. Using methods well know to those skilled in the art, a portion of the polymeric material hydroxyl groups can be derivatized to introduce polar groups, for example carboxylic acid esters, such as benzoate esters; phosphate esters; ethers, such as phenyl ethers; and sulfonic acid esters, such as methyl sulfonates, phenyl sulfonates, p-toluene sulfonates (tosylates), and p-bromophenyl sulfonates (brosylates).
Derivitization of the hydroxyl groups of the polymeric material increases its molecular weight and reduces the number of hydroxyl groups, typically reducing both the solubility and the rate of dissolution of the polymeric material in the developer. Although is important that the level of derivitization be high enough that the polymeric material acts as a solubility-suppressing component, it should not be so high that, following thermal imaging, the polymeric material is not soluble in the developer. Although the degree of derivitization required will depend on the nature of the polymeric material and the nature of the moiety containing the polar groups introduced into the polymeric material, typically about 0.5 mol % to about 5 mol %, preferably about 1 mol % to about 3 mol %, of the hydroxyl groups will be derivatized. These derivatized polymeric materials can act as both the second polymeric material and a solubility-suppressing component. They can be used alone in the top layer, or they can be combined with other polymeric materials and/or solubility-suppressing components.
One preferred group of polymeric materials that comprise polar groups and function as solubility-suppressing components are derivatized phenolic polymeric materials in which a portion of the phenolic hydroxyl groups have been converted to sulfonic acid esters, preferably phenyl sulfonates or p-toluene sulfonates. Derivitization can be carried by reaction of the polymeric material with, for example, a sulfonyl chloride such as p-toluene sulfonyl chloride in the presence of a base such as a tertiary amine. A preferred polymeric material is a derivatized novolac resin in which about 1 mol % to 3 mol %, preferably about 1.5 mol % to about 2.5 mol %, of the hydroxyl groups have been converted to phenyl sulfonate or p-toluene sulfonate (tosyl) groups.
It will be appreciated by those skilled in the art that although phenolic polymers which have been derivatized with polar groups [e.g., polymers in which some of the hydroxyl groups have been derivatized with sulfonic acid ester groups or with groups that contain the diazonaphthoquinone moiety] are soluble in aqueous alkaline developer, a layer comprising or consisting essentially of one or more of these materials is xe2x80x9cinsolublexe2x80x9d in aqueous alkaline developer. This is because solubility and insolubility of the layer are determined by the relative rates at which the imaged and unimged regions of the layer are removed by the developer. Following imagewise thermal exposure of a layer comprising or consisting essentially of one or more of these derivatized phenolic polymeric materials, the exposed regions of the layer are removed by the aqueous alkaline developer more rapidly than the unexposed regions. If the development step is carried out for an appropriate time, the exposed regions are removed and the unexposed regions remain, so that an image made up of the unexposed regions is formed. Hence the exposed regions are xe2x80x9cremovablexe2x80x9d or xe2x80x9csolublexe2x80x9d in the aqueous developer and the unexposed regions are xe2x80x9cnot removablexe2x80x9d or xe2x80x9cinsolublexe2x80x9d in the aqueous alkaline developer.
The solubility-suppressing components are believed not to be sensitive, i.e. photoreactive, themselves to radiation in the range of about 600 nm to about 800 nm and radiation in the range of about 800 nm to about 1200 nm, the range typically used for imaging a thermally imageable element.
If radiation is to be used for imaging and it is to be absorbed in the underlayer (i.e., the underlayer comprises an imaging radiation absorber), the solubility-suppressing component preferably should not absorb a significant amount of the imaging radiation. The imaging radiation should pass through the top layer so that it is absorbed by the absorber in the underlying underlayer. Thus, unless absorption of imaging radiation by the top layer is desired, when a dye is used as the solubility-suppressing component, it should not absorb a significantly at the imaging wavelength. Preferably, the imaging radiation absorber absorbs more strongly in the range of about 800 nm to about 1200 nm than it does in the visible (i.e., about 380 nm to about 780 nm).
If absorption of radiation in the range of 600 nm to 1200 nm, or the range of 800 nm to 1200 nm, in the top layer is desired, an absorber, i.e. a photothermal conversion material such as is described above, may be added to the top layer. When the underlayer contains a photothermal conversion material, the top layer may also contain a photothermal conversion material, which typically is the same infrared absorbing material used as the photothermal conversion material in the underlayer. When the underlayer comprises a negative-working, base soluble photosensitive composition, the photothermal conversion material may be in the top layer, in the underlayer, or in both the top layer and the underlayer.
When a photothermal conversion material is present in the top layer, it may comprise infrared absorber or dye bound to a phenolic material (i.e., a phenolic material derivatized with an infrared absorber or infrared absorbing dye). If an appropriate infrared absorber or dye is selected, the derivatized polymeric material can act as the second polymeric material, the solubility-suppressing component, and/or the photothermal conversion material.
The top layer may also comprise dye to aid in the visual inspection of the exposed and/or developed element. Printout dyes to distinguish the exposed regions from the unexposed regions during processing. Contrast dyes distinguish the unimaged regions from the imaged regions in the developed plate.
The top layer may also comprise a cellulose polymer or polymers to improve the resistance of the layer to blanket washes with, for example, petroleum ethers, glycols, glycol ethers, and branched alkanols, for example iso-propyl alcohol and 1-methoxypropan-2-ol. Preferably the top layer comprises a carboxylic acid containing cellulose polymer, such as cellulose acetate phthalate, cellulose acetate hydrogen phthalate, and/or cellulose acetate trimellitate. Typically the cellulose polymer comprises 0.1% to 50% by weight, preferably 5% to 20% by weight, and more preferably 8% to 12% by weight of the top layer. The acid number for the cellulose polymer is typically 50-210, preferably 100-180.
The top layer and/or the underlayer may comprise Particulate material to improve both the scratch resistance and presslife of the exposed and developed element. Typically, the particles have an average diameter about 0.5 xcexcm and about 10 xcexcm. Organic and/or inorganic particulate material may be used. Examples of organic particles are crosslinked polystyrene beads and polymethyl methacrylate beads. Non-crosslinked polymers such as polycarbonate and acrylonitrile-butadiene copolymers, which form particles in situ during the drying process, are also useful in improving presslife of the exposed and developed elements. Inorganic particles include, for example, silica and alumina particles. The amounts of particles used can range from 0.5% to 30% of the coating weight of the layer, preferably is 5% to 15%.
The thermally imageable element may be prepared by sequentially applying the underlayer over the hydrophilic surface of the hydrophilic substrate, and then applying the top layer over the underlayer using conventional coating or lamination methods. However, it is important to avoid intermixing the underlayer and top layer.
The underlayer, or first layer, may be applied over the hydrophilic substrate by any conventional method. Typically the ingredients are dispersed or dissolved in a suitable coating solvent, and the resulting mixtures coated by conventional methods, such as spin coating, bar coating, gravure coating, or roller coating. The top layer, or second layer, may be applied over the underlayer, typically to the surface of the underlayer by any conventional method, such as those listed above. The term xe2x80x9csolventxe2x80x9d includes mixtures of solvents, especially mixtures of organic solvents.
Selection of the solvents used to coat the underlayer and to coat the top layer will depend on the nature of the first polymeric material, the second polymeric material, and the other ingredients present in the layers. To prevent the underlayer from dissolving and mixing with the top layer when the top layer is coated over the underlayer, the top layer should be coated from a solvent in which the first polymeric material is essentially insoluble. Thus, the coating solvent for the top layer should be a solvent in which the second polymeric material is sufficiently soluble that the top layer can be formed and in which the first polymeric material is essentially insoluble. Although the solvents used depend on the nature of the polymeric materials, typically the first polymeric material will be soluble in more polar solvents and insoluble in less polar solvents so that the solvent used to coat the underlayer is more polar than the solvent used to coat the top layer. Consequently, the top layer can typically be coated from a conventional organic solvent such as toluene, 2-butanone, propyleneglycol acetate, n-butanol, iso-propyl alcohol, or butyl acetate. An intermediate drying step, i.e., drying the underlayer to remove coating solvent before coating the top layer over it, may also be used to help prevent mixing of the layers.
The top layer may be coated as an aqueous dispersion to avoid dissolving the underlayer during the coating process. Alternatively, the underlayer, the top layer or both layers may be applied by conventional extrusion coating methods from a melt mixture of layer components. Typically, such a melt mixture contains no volatile organic solvents.
Imaging of the imageable element produces an imaged element, which comprise a latent image of imaged and unimaged regions. Developing the exposed element to form a developed element converts the latent image to an image by removing the exposed regions of the top layer and the underlayer, and exposing the hydrophilic surface of the underlying substrate. The element is positive working, in that the underlayer and top layers are removed in the exposed regions. The exposed regions become the non-ink accepting regions.
While not being bound by any theory or explanation, it is believed that a thermally frangible complex is formed between the solubility-suppressing component and the polymeric material. When the element is heated, typically be imagewise thermal exposure, the thermally frangible complex breaks down. The developer penetrates the exposed regions of the top layer much more rapidly than it penetrates the unexposed regions. The underlying regions of the underlayer are removed along with the exposed regions of the top layer, revealing the underlying hydrophilic surface of the substrate.
The thermally imageable element may be imaged with a laser or an array of lasers emitting modulated near infrared or infrared radiation in a wavelength region that is absorbed by the element. Infrared radiation, especially infrared radiation in the range of about 800 nm to about 1200 nm, is typically used for imaging a thermally imageable element. Imaging is conveniently carried out with a laser emitting at about 830 nm or at about 1056 nm. Suitable commercially available imaging devices include image setters such as a Creo Trendsetter (available from the CREO Corp., British Columbia, Canada) and a Gerber Crescent 42T (available from the Gerber Corporation).
Alternatively, the thermally imageable element may be imaged using a conventional apparatus containing a thermal printing head. An imaging apparatus suitable for use in conjunction with the imageable elements includes at least one thermal head but would usually include a thermal head array, such as a TDK Model No. LV5416 used in thermal fax machines and sublimation printers. When exposure is carried out with a thermal head, it is unnecessary that the element absorb infrared radiation. However, elements that absorb infrared radiation can be imaged with a thermal head.
In either case, the image signals are stored as a bitmap data file on a computer. Such files may be generated by a raster image processor (RIP) or other suitable means. For example, a RIP can accept input data in page-description language, which defines all of the features required to be transferred onto the imageable element, or as a combination of page-description language and one or more image data files. The bitmaps are constructed to define the hue of the color as well as screen frequencies and angles.
The developer may be any liquid or solution that can penetrate and dissolve both the exposed regions of the top layer and the underlying regions of the underlayer without substantially affecting the complimentary unexposed regions. While not being bound by any theory or explanation, it is believed that image discrimination in these systems is based on a kinetic effect. The exposed regions of the top layer dissolve more rapidly in the basic developer than the unexposed regions. Development is carried for a long enough time to dissolve the exposed regions of the top layer and the underlying regions of the underlayer in the developer, but not long enough to dissolve the unexposed regions of the top layer. Hence the exposed regions are described as being xe2x80x9csolublexe2x80x9d in the developer because they dissolve more rapidly in the developer than the unexposed regions. However, it will be appreciated that, if the exposed element is developed for a long enough time, both the exposed and the unexposed regions will be dissolved.
Useful developers are aqueous solutions having a pH of about 7 or above. Preferred aqueous alkaline developers are those that have a pH between about 8 and about 13.5, typically at least about 11, preferably at least about 12. Wholly aqueous developers, i.e., those that do not comprise an added organic solvent, are preferred. Useful developers include commercially available developers, such-as PC3000, PC955, and PC9000, aqueous alkaline developers each available from Kodak Polychrome Graphics LLC.
Typically the developer is applied to the imaged element by rubbing or wiping the top layer with an applicator containing the developer. Alternatively, the imaged element may be brushed with the developer or the developer may be applied to the element by spraying the top layer with sufficient force to remove the exposed regions. In either instance, a developed element is produced.
The developed element, typically a lithographic printing plate or printing member, comprises (1) regions in which the underlayer and top layer have been removed revealing the underlying surface of the hydrophilic substrate, and (2) complimentary regions in which the underlayer and top layer have not been removed. The regions in which both the underlayer and top layer have not been removed are ink receptive and correspond to the regions that were not exposed during imaging.
If desired, a post-development baking step can be used to increase the run length of the plate. Baking can be carried out, for example, at about 220xc2x0 C. to about 240xc2x0 C. for about 7 to 10 minutes.
If the underlayer comprises a negative-working, base soluble photosensitive composition, the developed element is exposed to actinic radiation. Any convenient source or sources of actinic radiation providing wavelengths in the region of the spectrum that overlap the absorption bands of the negative-working, base soluble photosensitive composition can be used to activate photoinsolubilization. By xe2x80x9cactinic radiationxe2x80x9d is meant any radiation that can induce photoinsolubilization in the underlayer. The radiation can be natural or artificial, monochromatic or polychromatic, incoherent or coherent. Conventional light sources include fluorescent lamps, mercury, metal additive and arc lamps. Coherent light sources are lasers, such as xenon, argon ion, and ionized neon lasers, as well as tunable dye lasers and the frequency doubled neodymium: YAG laser.
The advantageous properties of the invention can be observed by reference to the following examples that illustrate, but do not limit, the invention.